Wereviper

Werevipers are venomous lycanthropes. Many resemble cobras, adders, and mambas, with the coloring to match. Werevipers prove to be adept trackers, for they can detect the scent of their own venom as it’s exuded from a recently bitten creature’s open wound. A wereviper shrinks in size when transfiguring into its beast form.

Wereviper CR: 4

Medium humanoid (any race, shapechanger), neutral evil

Armor Class: 13

Hit Points: 39 (6d8+12)

Speed: 30 ft , swim: 20 ft , climb: 10 ft

STR

13 +1

DEX

15 +2

CON

14 +2

INT

10 +0

WIS

11 +0

CHA

13 +1

Skills: Acrobatics +6, Perception +4, Stealth +4

Damage Immunities: bludgeoning and piercing from nonmagical weapons that aren’t fashioned from mongoose fangs; slashing from nonmagical weapons that aren’t silvered

Senses: blindsight 10 ft. (beast or hybrid form only), darkvision 60 ft., passive Perception 14

Languages: any one language (can't speak in beast form)

Challenge Rating: 4

Proficiency Bonus: +2

Shapechanger. The wereviper can use its action to polymorph into a snake-humanoid hybrid or into a Small snake, or back into its true form, which is humanoid. Its statistics, other than its size and AC, are the same in each form. Any equipment it is wearing or carrying isn’t transformed. It reverts to its true form if it dies.
Keen Smell. The wereviper has advantage on Wisdom (Perception) checks that rely on smell.
Tracker. The wereviper has advantage on Wisdom checksmade to detect or track creatures that it has bit in the last hour. Constructs and elementals are unaffected by this trait.

Actions

Multiattack (Humanoid or Hybrid Form Only). The wereviper makes two attacks.
Bite (Beast or Hybrid Form Only). Melee Weapon Attack: +4 to hit, reach 10 ft., one target. Hit: 6 (1d6 + 2) piercing damage and the target must make a DC 12 Constitution saving throw, taking 7 (2d6) poison damage on a failed save, or half as much damage on a successful one. If the target is a humanoid, it must succeed on a DC 12 Constitution saving throw or be cursed with wereviper lycanthropy.
Dagger (Humanoid or Hybrid Form Only). Melee Weapon Attack: +4 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 4 (1d4 + 2) piercing damage.